There is a continuing need for improvements to microfluidic devices, in particular for biological so-called lab-on-chip applications. Such a device may comprise, for example, a section for generating biological material to be analysed, a section for manipulating the biological material, and a sensing section to sense the results, all integrated within a single device. Here we are particularly concerned with sensing biological particles such as spores, cells and similar entities. Various prior art sensing techniques are known including that described in U.S. Pat. No. 6,352,838, in which a dielectrophoretic force is employed to selectively separate a target material from a contaminant material; and that described in US2007/0175755 (which employs a fluid flow generally aligned with a longitudinal axis of a fluid containing cell). Further background prior art can be found in: US 2004/163955; U.S. Pat. No. 5,858,192; WO2004/059290; US2002/088712; US2007/175755; and GB2266153A.